Since joey45 is offline I hope he won't mind me answering you. Here's a link to a good explanation of what a 'bullseye' is. In short, it's an arbitrary place from which things (like enemy reports) get measured, so that the enemy can't work out where your are by listening into radio transmissions.
So instead of saying you, or an enemy you've spotted are 3 miles north of a certain town, all the friendlies are given a secret bullseye location and reference things from that.
The bullseye on the map won't necessarily have anything to do with your mission objectives, and probably won't guide you to bad guys.
http://falcon4.wikidot.com/concepts:bullseye
Hope this helps
smith